MEXICALI, Mexico — Arrests of illegal migrants trying to cross the southern U.S. border have plummeted to levels not seen since the early 1970s, according to tallies released by the Department of Homeland Security last week, a historic shift that could reshape the debate over immigration reform.

The Border Patrol apprehended 327,577 illegal crossers along the U.S.-Mexico border in fiscal year 2011, which ended Sept. 30, numbers not seen since Richard Nixon was president, and a precipitous drop from the peak in 2000, when 1.6 million unauthorized migrants were caught.

More than 90 percent of the migrants apprehended on the southwest border are Mexican. The number of illegal migrants arrested at the border has been dropping over the past few years but appears to be down by more than 25 percent this year.

What is needed now is for the U.S. Border Patrol to return to its interior enforcement responsibilities long exercised in the past but neglected of late, including farm and ranch inspections, employer sanctions raids, interior transportation hub checks, and a reopening of interior Border Patrol Stations, such as Tracy, CA, that were closed down during the Jorge Bush Administration.